Insights into the formation of hydroxyl ions in calcium carbonate: temperature dependent FTIR and molecular modelling studies

Abstract

This paper describes the use of temperature dependent FTIR spectroscopy and molecular modelling studies to establish the origin and the nature of surface hydroxyl ions on calcium carbonate. It has been demonstrated that two types (Type I, corresponding to a band at 3690 cm⁻¹ and Type II, corresponding to a band at 3640 cm⁻¹) of hydroxyl ions exist on calcium carbonate surfaces prepared by the carbonation method. Type I hydroxyl ions are ascribed to those of the unreacted calcium hydroxide (portlandite) present due to incomplete carbonation and Type II hydroxyl ions are ascribed to interstitial defects which are strongly associated with the calcium carbonate lattice framework. Interestingly, the calcium carbonate samples prepared by the solution method do not possess Type I/Type II hydroxyl ions. A molecular modelling exercise was carried out to generate the calcite 104 plane, and the different modes of adsorption of water on the calcite 104 plane were derived based on energy minimisation calculations. The possibility of replacement of a carbonate ion either by (i) two hydroxyl ions or (ii) a hydroxyl and a bicarbonate ion has been considered. The replacement of a carbonate ion by one hydroxyl and one bicarbonate ion is indicative of the presence of surface/interstitial defects on calcite (corresponding to Type II hydroxyl ions assigned by FTIR studies). A molecular description of hydroxylating calcite surfaces is discussed in detail and the results from the energy of formation at zero water coverage corroborate the above findings. The calculations also predict the formation of a maximum of two pairs of hydroxyl and bicarbonate ions over a surface area of 1.0 nm², during chemisorption at low surface coverages.